The present invention relates to a drill bit that is intended to be included in a drill string for top hammer drilling. The drill string includes a set of central rod members for transferring impact energy to the drill bit from a top hammer, and a set of tubular members surrounding the set of rod members for transferring rotation to the drill bit. The invention also relates to a tubular member and a drill string.
From Lundstrom et al U.S. Pat. No. 4,094,364, a device of the above-mentioned kind is previously known. The set of central rod members comprises relatively massive rods, and flushing medium is fed to the drill bit via an annular space formed between the set of rod members and the set of tubular members. The drill bit is closed at its rear or upper impact-receiving end. A central channel for flushing fluid begins forwardly of the closed rear end of the drill bit and communicates with the annular channel by means of a transverse passage to receive flushing medium and discharge that medium at the front of the drill bit.
The reason for closing the rear end of the drill bit is that the energy transferring surface of the rods and drill bit must be sufficiently large in order to make possible the use of top hammer equipment that generates enough energy to achieve a satisfying drill penetration rate. For drills which make larger drill hole diameters, especially exceeding 125 mm, the rod members have a sufficiently large diameter to accommodate an internal flushing channel. Such a channel extends longitudinally completely through each rod and communicates with a channel which extends longitudinally completely through the drill bit. However, such an internal flushing channel provides insufficient lubrication for the splined coupling between the drill bit and the tubular member. Furthermore, in the case where the drill bit comprises loosely abutting pieces, the flushing medium traveling through the flushing channel may, as it jets past the loose parting line between the drill bit pieces, create a suction in the space disposed outside of the drill bit. Consequently, cuttings (crushed rock) at the front of the drill bit can be sucked into the splined coupling and damage same.
An object of the present invention is to provide a drill bit and a tubular member included in a drill string as described above, cooperating to eliminate the problems discussed above.